This disclosure relates to methods and apparatus for facilitating the accurate measurement of frequency shift induced by the Doppler effect on a signal wave as a result of relative linear motion between the source (transmitter or reflector) and the receiving sensor.
It is highly desirable to be able to operate on a one-way propagated signal wave from a remote source to extract not only the data/information it carries as a result of deliberate modulation at the source, but also changes in any of its parameters, introduced after the signal energy has left the source, that may provide information about the propagation mechanism or about position and derivatives of position coordinates of the source relative to the position of the receiving sensor.
Of particular importance in this disclosure is change, or shift, in the carrier frequency of a signal wave imparted by the Doppler effect, in propagation, due to relative linear motion between source and receiver sensing element. Such relative linear motion may exist between source platform and receiver platform, which is manifested by a rate of change of range, or separating radial distance, between the platforms. Such so-called range rate gives rise to what, for distinction, we call herein range-rate Doppler shift. Relative linear motion may also be introduced independently in situ (or within the structure of one or both of the terminals, or on-site, or onboard) by locally moving the transmitting antenna means or/and the receiving sensor means, in actuality or by simulation means. This latter gives rise to what we call induced Doppler (or directional) frequency modulation, or IDFM for short.
Accurate measurement of range-rate Doppler enables the determination of range rate (or relative velocity) between source and receiver platforms and, with proper initiation and integration of the measured range rate, the determination of "instantaneous" range or radial distance between the two, without requiring the addition of a coherent transponder at the source to be tracked and signal radiation from the station of the receiving sensor.
In air traffic surveillance, monitoring and control, in radio-beacon aids to navigation and position location, and in aircraft, missile and weapons test ranges, the possibility of deriving trackmetric data from a communication signal, a beacon signal or a telemetry signal emanating from the target object obviates the need for adding a special coherent transponder onboard the target aircraft, "lighthouse", or test object for coherent turn-around re-radiation of a sensor-sourced illuminating search signal. A coherent transponder for a special tracking search signal adds an oft undesirable or unacceptable requirement of additional power-consuming, space-occupying, weight-increasing and hardware reliability degrading equipment, not to mention the tracking signal sourcing and reception requirements in the search and sensing station.
The need for an onboard transponder can of course be obviated in some instances by illuminating the target object from the search and sensing station and measuring the two-way Doppler on the resulting back-scattered or skin-reflected signal. However, skin-returned signals are subject to a) two-way, 1/.rho..sup.4 -law propagation attenuation, where .rho. is the one-way range; b) limitations of small physical dimensions, and hence reflection cross-section, of most target objects; and c) the limitations of skin-reflection lobing and other scattering (or "radar") cross-section degrading effects.
It is therefore an object of this invention to provide methods for facilitating range-rate Doppler frequency-shift measurements on one-way propagated signals that are intended primarily for communication, telemetry or location beaconing.
However, while the source-applied modulation can usually be detected without serious interference from the Doppler-shift waveform, the Doppler frequency-shift would ordinarily be severely masked by any source modulation of the signal frequency.
Accordingly, it is a principal object of this invention to provide a method for effectively suppressing frequency modulation applied at the source while retaining any range-rate Doppler frequency-shift/modulation present to within a specified tolerance on error due to residual source modulation and/or Doppler-shift modulation distortion.
This invention also relates to the reception and radiation of radio waves in such ways as to induce in them direction-dependent frequency shifts, henceforth referred to as "IDFS", by means of the Doppler effect, free of certain phase-step and/or frequency-step modulations that degrade the measurement of said IDFS, and thereby facilitate the accurate measurement of said IDFS. It is well-known that said ways comprise in situ (or on-site) controlled motion of one or more than one antenna, or the simulation of such motion by commutating the receiver input, or the transmitter output, among fixed, spaced antennas arranged along an antenna line of motion (henceforth, LOM). The advantages of employing actual or simulated in situ moving antennas are well known in the art (see Baghdady U.S. Pat. Nos. 4,060,809, 4,088,955, 4,106,023, 4,203,113). It is also known that the dimensional limitations of platforms necessitate either jumps in antenna position from one end of an LOM to another, which introduces unwanted phase steps, or the abrupt reversal of the sense of antenna motion along the LOM, which introduces frequency steps, in between gradual excursions of antenna position between the end points of the LOM. It is further known that if further measures as provided by the present invention are not included in the implementation of said methods of inducing IDFS, to counteract the occurrence or eliminate said phase steps and/or frequency steps, then the IDFS measurement will fall short of achieving the full potential performance described in the reference patents, for direction-of-arrival measurement, emitter location-coordinates determination, radio aids to navigation (including ultra-precision rectilinear Doppler VOR) air traffic control instrumentation, multipath separation, and cochannel- and adjacent-channel interference rejection, as well as collision anticipation miniradar for automobile airbag inflation and headway control, and spacecraft-docking miniradar aid.
It is therefore a further object of this invention to provide methods and apparatus for cancelling out phase steps and/or frequency steps due to abrupt changes in antenna position or sense of in situ antenna motion intended to induce signal wave IDFS.
It is yet a further object of this invention to provide methods and apparatus for eliminating unwanted phase-step and/or frequency-step modulation components present on a signal wave.
These and other objects and features of this invention will become apparent from the claims, and from the following description when read in conjunction with the accompanying drawings.